Next-generation force test cell designs reveal a set of recurring scalability challenges that arise when laboratory-scale mechanical measurement must translate into reproducible, high-throughput research workflows. Understanding these challenges requires first establishing what force test cells measure, how that measurement connects to electrochemical behaviour, and where design decisions begin to constrain or enable scale.
This article builds from foundational definitions through to practical application, covering the mechanical principles, design trade-offs, and workflow implications that battery materials researchers encounter when working with force-sensitive electrochemical test cells.
What is a force test cell and what does it measure?
A force test cell is an electrochemical test cell equipped with a load sensor that measures the mechanical force exerted by electrode materials as they expand and contract during cycling. Rather than tracking electrochemical signals alone, it captures the mechanical response of the cell stack in real time.
During lithium intercalation and deintercalation, electrode materials undergo volumetric changes. In graphite anodes, for example, full lithiation produces roughly a ten percent volume increase. In silicon-containing anodes, this expansion is far more pronounced. A force test cell quantifies the stress these changes generate within a constrained cell geometry, providing data that is directly relevant to understanding mechanical degradation, delamination, and cycle-life behaviour.
The measurement output is typically force in Newtons or pressure in MPa, recorded alongside standard electrochemical parameters such as voltage, current, and capacity. This dual-channel approach makes force test cells particularly useful for correlating mechanical events with electrochemical phenomena, including capacity fade, overpotential increases, and changes in coulombic efficiency.
How mechanical stress and electrochemical performance are linked
Mechanical stress within a cell is not merely a structural concern. It directly influences the electrochemical interfaces that govern performance, particularly the Solid Electrolyte Interphase (SEI) layer that forms on the anode surface during initial cycling.
When electrode particles expand and contract repeatedly, the SEI layer is placed under cyclic mechanical stress. Cracking or delamination of the SEI exposes fresh anode surface to the electrolyte, triggering renewed SEI formation and consuming lithium irreversibly. This manifests as a measurable reduction in coulombic efficiency over successive cycles.
In solid-state battery testing, where a solid electrolyte is used in place of a liquid, the mechanical coupling between the electrode and electrolyte is even more direct. Insufficient or uneven stack pressure can create interfacial voids that increase contact resistance and raise the overpotential required to drive lithium-ion transport. Conversely, excessive pressure can fracture brittle ceramic electrolyte pellets. Force test cells such as the PAT-Cell-Solid provide the data needed to identify and maintain the optimal pressure window for solid-state systems.
Key design features that define next-generation force test cells
Next-generation force test cells are distinguished by several design features that improve measurement precision, experimental flexibility, and compatibility with advanced battery chemistries.
Integrated load sensing
Earlier approaches to mechanical measurement often relied on external load frames separate from the electrochemical cell, and conventional test cells do not include a force sensor at all — only the initial pressure is read, meaning that mechanical settling can reduce it over time without detection. Modern designs such as those from EL-CELL integrate the load sensor directly into the cell housing, reducing compliance in the measurement chain and improving the accuracy of force readings. The integrated force sensor in EL-CELL cells continuously tracks load changes throughout cycling, and an optional gas pressure sensor can be added to measure force changes caused by gas evolution separately from mechanical ones. This integration also simplifies the experimental setup and reduces the risk of misalignment between mechanical and electrochemical measurement axes.
Controlled and adjustable stack pressure
For solid-state battery testing, the ability to set and maintain a defined stack pressure throughout cycling is essential. Next-generation cells such as the PAT-Cell-Force incorporate spring-loaded or screw-adjustable mechanisms that allow researchers to apply a specific preload and monitor how that load evolves as the electrode stack changes thickness. This is particularly important when characterising solid electrolytes under realistic operating pressures.
Homogeneous electrode compression
Conventional test cells compress electrode material inhomogeneously, introducing variability that undermines measurement reproducibility. The PAT-Solid-Core insert, used in both the PAT-Cell-Force and the PAT-Cell-Solid, addresses this directly through guided plane-parallel tungsten carbide plungers and a dedicated pressing tool that ensure homogeneous compression across the entire electrode area. These tungsten carbide plungers also withstand high mechanical loads without embedding particles into their surface — a problem that affects conventional plungers, which must be ground or polished between measurements and gradually alter cell geometry as a result.
Sealing materials and moisture contamination
Conventional test cells are typically sealed with O-rings and often use PEEK housings, which absorb significant moisture and require drying at 120°C under vacuum to reduce contamination risk. EL-CELL cells use aluminum seals and glass-metal feedthroughs instead of O-rings, and PPS plastic instead of PEEK. PPS absorbs considerably less moisture than PEEK, reducing contamination risk and shortening preparation time without the need for high-temperature vacuum drying.
Compatibility with in-situ and operando techniques
Many next-generation force test cells are designed to be compatible with additional measurement modalities, including electrochemical impedance spectroscopy (EIS), dilatometry, and optical access. This multi-modal capability allows researchers to correlate force data with impedance spectra or thickness changes recorded simultaneously, building a more complete picture of cell behaviour. Instruments such as the ECD-4-nano extend this approach by combining dilatometric and electrochemical measurement within a single platform.
Where scalability challenges emerge in force test cell design
Scalability in the context of force test cells refers to two related but distinct problems: scaling measurement accuracy across a range of electrode thicknesses and active areas, and scaling experimental throughput across multiple parallel test channels.
Assembly reliability and preparation consistency
Before measurement accuracy can be considered, cells must first be assembled successfully. Conventional test cells have a high assembly failure rate — studies cite 43% — meaning that even experienced builders achieve only around 4 out of 5 working cells, while inexperienced ones fall below 50%. This level of attrition is a significant barrier to throughput at scale. The PAT-Cell-Force and PAT-Cell-Solid from EL-CELL standardize and simplify preparation so that nearly every cell runs without failure, making them far better suited to high-throughput research workflows.
Electrode area and force uniformity
As electrode area increases, maintaining uniform pressure distribution across the entire active surface becomes progressively more difficult. Small misalignments in the cell stack, variations in electrode coating thickness, or slight deviations in separator uniformity all produce localised pressure gradients. At small electrode areas, these gradients are minor. At larger areas, they introduce systematic measurement error and can cause non-uniform electrochemical utilisation across the electrode surface.
Sensor resolution versus dynamic range
Force sensors face an inherent trade-off between resolution and dynamic range. A sensor calibrated to measure small force changes with high resolution will saturate if the electrode generates unexpectedly large mechanical excursions. Conversely, a sensor with a wide dynamic range may lack the resolution to detect subtle force events that precede mechanical failure. Selecting the correct sensor specification for a given electrode chemistry requires prior knowledge of the expected force regime, which is not always available when characterising novel materials.
Thermal management at scale
Temperature affects both the mechanical properties of electrode materials and the force readings themselves, since thermal expansion of the cell hardware contributes a background signal that must be subtracted from the electrode-generated force. In single-channel setups, this correction is straightforward. In multi-channel systems operating at different temperatures simultaneously, thermal cross-talk between channels and differential hardware expansion become sources of measurement uncertainty that are difficult to eliminate entirely.
How design trade-offs shape scalability decisions
Building on the scalability challenges described above, the design decisions that researchers and instrument developers face when configuring force test cell systems are rarely straightforward optimisations. They involve genuine trade-offs between competing requirements.
One central trade-off is between cell rigidity and measurement sensitivity. A stiffer cell housing constrains electrode expansion more completely, which produces a larger, more easily measured force signal. However, the constrained boundary condition does not replicate the free-swelling behaviour of a pouch cell or the lightly constrained environment of a prismatic cell. Researchers must decide whether they are characterising intrinsic material properties, which may favour a stiffer constraint, or mimicking realistic operating conditions, which may require a softer or pressure-controlled boundary.
A second trade-off involves cell footprint and parallelisation. Smaller cells are easier to handle, require less active material, and can be packed more densely into a multi-channel test station. However, smaller electrode areas amplify edge effects and reduce the statistical representativeness of the measurement. Larger electrode areas improve representativeness but increase material consumption and reduce the number of parallel experiments that fit within a given instrument capacity.
A third consideration is the balance between standardisation and flexibility. Standardised cell geometries simplify data comparison across experiments and between laboratories. Flexible or modular designs allow researchers to adapt the cell to non-standard electrode formats or to incorporate additional sensors. Next-generation force test cells increasingly attempt to offer both through modular hardware architectures, as exemplified by the PAT Series approach, though this adds mechanical complexity and potential sources of variability.
Applying force test cell insights to real battery development workflows
Force test cell data is most valuable when it is integrated into the broader experimental workflow rather than treated as a standalone measurement. In practice, this means aligning force measurements with electrochemical cycling protocols, dilatometry data, and post-mortem analysis.
For electrode development, force measurements during the first few formation cycles can identify whether a new binder formulation or coating density is mechanically stable before committing to longer cycle-life studies. A sharp increase in force during the first lithiation, followed by incomplete force recovery on delithiation, is an early indicator of irreversible mechanical deformation.
For solid-state battery development, force test cells provide direct feedback on the stack pressure required to maintain adequate interfacial contact throughout cycling. This information is essential for designing the mechanical housing of prototype solid-state cells and for specifying the stack pressure that must be maintained in any future scaled-up format.
In quality control workflows, force signatures can serve as a rapid screening tool. Electrodes or cell stacks that deviate from a reference force profile can be flagged for further investigation without waiting for full cycle-life data. This shortens the feedback loop between material synthesis and performance characterisation.
Connecting force data to EIS measurements adds another layer of interpretive depth. Changes in interfacial resistance measured by EIS can be correlated with specific force events, helping to distinguish between resistance increases caused by SEI growth, electrode delamination, or electrolyte decomposition. The PAT-Tester-i-16 supports this workflow by providing integrated EIS capability alongside multi-channel electrochemical cycling.
How EL-Cell GmbH supports force and solid-state battery testing
EL-Cell GmbH designs and manufactures test cells and instruments that address the measurement challenges described throughout this article. Our product range includes dedicated solutions for force measurement and solid-state battery testing, with the following capabilities:
- The PAT-Cell-Force is a force-sensing test cell that measures stack pressure continuously during electrochemical cycling, with a design optimised for reproducible electrode compression and integration with standard PAT Series hardware.
- The PAT-Cell-Solid is designed specifically for solid-state battery testing, providing controlled and measurable stack pressure for pelletised solid electrolyte systems.
- The PAT-Cell-Press allows researchers to apply defined uniaxial pressure to cell stacks, supporting experiments that require precise mechanical boundary conditions.
- The PAT-Tester-i-16 provides up to 16 independent test channels with integrated EIS capability and temperature-controlled cell chambers, enabling parallel force and electrochemical measurements at scale.
- All hardware is designed as part of an interoperable ecosystem, so force, impedance, and dilatometry data can be collected in a single experimental setup without compatibility issues between instruments.
Researchers working on electrode mechanics, solid electrolyte characterisation, or mechanical degradation studies are welcome to contact our Application Laboratory directly to discuss experimental requirements and instrument configurations suited to their specific workflows.
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